The key is discovering value and avoiding waste.
Building Information Modeling (BIM) is used extensively throughout the construction industry, with some trades fabricating directly from BIM as standard practice. Even steel rebar for cast-in-place concrete is being modeled and fabricated using BIM. So why isn’t all Cold-Formed Steel (CFS) framing included in a model, and why do some CFS contractors consider BIM a waste of time?
What Makes CFS BIM Unique?
Unlike most structural materials, the application of CFS framing on a project often varies significantly. For many projects, the CFS framing is a non-bearing component like interior walls/ceilings or exterior envelope framing, where many member locations are highly adjustable in the field. However, in other projects, the CFS framing represents part of the primary structural system where member locations are critical. Therefore, the strategic use of CFS BIM needs to acknowledge these differences.
Additionally, the volume of parametric data used for CFS framing models can be significant. For example, with the spacing of CFS framing at 16 or 24 inches on-center, CFS models can require thousands of model objects. If connectors and fasteners are required to be modeled, the number of objects can quickly increase two-fold. While the latest software can automatically populate CFS framing into wall objects, the model volume and maintenance can still be costly. So, when is the reward worth the cost?
CFS Modeling and VDC
It is important to understand the BIM terms most commonly used by design teams and Virtual Design and Construction (VDC) teams. Most contracts requiring BIM deliverables include a BIM Execution Plan (BXP). The BXP typically makes reference to the BIM Forum Level of Development (LOD) Specification for the required amount of modeling and detail for every scope involved. Below are the LOD specifications for CFS framing:
- LOD200 – Generic geometry modeled to represent an overall CFS assembly or system, approximate in terms of overall size and shape. (Modeling of individual CFS framing members is not required.)
Allows for minimal trade coordination and only very basic takeoff. - LOD300 – Geometric objects modeled to represent an overall CFS assembly or system, accurate in terms of actual overall size and shape, including nominal openings. (Modeling of individual CFS framing members is not required.)
Allows for minimal trade coordination and basic takeoff. - LOD350 – Critical CFS framing modeled as individual elements for coordination purposes. Openings are modeled to actual rough-opening dimensions and include critical framing members such as jambs, headers, and sills. Diagonal kicker braces are also modeled for coordination purposes. Typical on-center infill framing may be omitted per a project’s BXP.
Allows for trade coordination with critical elements and standard takeoff. - LOD400 – All CFS framing and connections are modeled as individual elements, accurate in terms of size, shape, and location.
Allows for comprehensive coordination, exact takeoff, and is fabrication-ready.
VDC teams are accustomed to mechanical, electrical, plumbing, and fire protection (MEPF) trades producing BIM models varying from LOD350 to LOD400, which facilitates the prefabrication of many of these systems. CFS framing is a unique player in the VDC coordination space, as many CFS contractors choose to field frame the CFS system. Field framing allows the CFS contractor to easily make adjustments in the field. Therefore, determining the adequate LOD for CFS BIM should always be the first step. Adhering to a lower-value LOD may help save the overall project time and money while still offering significant coordination benefits.
Note: While LOD400 may be required for MEPF systems, the CFS framing may be best utilized at an LOD350 without the infill framing.
General Value for Engineers and Contractors
CFS BIM can provide CFS engineers and contractors with useful information to help with critical decision analysis, add value through enhanced coordination, and help to ensure better quality control. Precise model-based material takeoffs, generation of shop drawings, and even running machinery at fabrication facilities are possible through CFS BIM implementation. The benefit of modeling complicated geometries in 3D can help determine how CFS framing can achieve a particular architecture and also serves as a very effective visual aid for collaboration with design teams and field crews. CFS BIM offers value to CFS engineers and contractors in various ways, but there is no rule book to follow when choosing which methods and techniques to adopt as standard practice. It is important to explore what works best case by case to achieve the best outcome.
Note: Deciding to implement BIM internally often requires a long-term outlook. Added quality control and coordination may not realize immediate tangible benefits, but long-term return on the investment can outweigh the upfront cost.
Field-Framed Non-Bearing Exteriors
In most buildings, minimal MEPF items run through or penetrate the CFS envelope framing. Thus, CFS BIM may not significantly impact external VDC team coordination efforts, especially if the CFS framing is field-installed. However, building exterior walls with significant MEPF can benefit from CFS BIM. Examples include process structures, data centers, and lab facilities.
Note: Useful model coordination items can include base and top-of-wall conditions, slab edge clearance, tight tolerance conditions, and wall layout against other trades.
Comparing a CFS framing model to the architect’s and structural engineer’s models is an easy way to uncover potential design issues. For example, soffits and overhangs are common locations where deeper structural members may be required but aren’t clearly shown in architectural sections or details. CFS BIM allows CFS engineers and contractors to develop solutions with the added assurance of fit and coordination.
Note: When working on a structural steel building, consider requesting the steel fabricator’s model and not the structural engineer’s model since the fabricator’s model represents the anticipated field condition. Similar considerations apply to concrete buildings; however, not all concrete gets modeled by the contractor.
Modeling diagonal kickers that extend into a building’s interior space can identify conflicts with MEPF systems and interior ceilings. Typically, kickers function as critical support members, and adjustment of their position or location can be limited. Identifying any issues before construction allows systems and designs to be altered without re-work in the field, saving everyone valuable time and money.
Field Framed Non-Bearing Interiors
BIM for non-bearing interior CFS framing primarily benefits the VDC team coordination process in buildings with dense MEPF systems. For coordination purposes, BIM models for interior non-bearing CFS framing are typically comprised of critical framing members only. These include jamb studs, headers, sills, end studs, and corner studs. This aligns with LOD350 and helps the MEPF trades identify where their systems need to be adjusted or re-routed away from critical CFS members.
Modeled objects for critical framing members do not have to be exact representations of the framing in the field. For example, rationalized geometry can represent the framing members to encapsulate the worst case. However, geometries must not be modeled overly conservatively, as this may lead to unnecessary coordination efforts from other involved parties.
Note: Jambs and headers can be modeled using a single worst-case size to simplify model insertion.
For top-of-wall conditions, especially fire-rated wall assemblies, MEPF must route their systems in a manner that does not impede the construction of the wall or prevent the assembly from functioning as intended. Modeling no-fly zones are a simple and effective way to coordinate and avoid framing issues without the extra effort of modeling all the individual framing members.
Note: Top-of-wall no-fly zones can be modeled with solid extrusions, or top-of-wall tracks can use an elongated flange to create the no-fly zone.
Field Framed Load-Bearing Structures
For some buildings, the primary structural system utilizes load-bearing CFS framing. Minimal field adjustments are allowed with this system, as load-bearing CFS framing usually takes priority over other building systems. However, this may not hold in all cases, as some individual MEPF systems cannot adjust. For example, HVAC ductwork can be too large to fit between stud bays resulting in additional framed openings needing to be engineered, and consideration must be taken to avoid MEPF conflicts with shear walls and strap bracing.
Note: Toilet, shower, and tub drains, especially on floor joists, must be carefully coordinated as these drains can only be relocated by reconfiguring the space.
There are situations where CFS BIM, while helpful, may not be cost-effective. The MEPF trades may not have the expertise or resources to provide models for certain projects. Additionally, if the MEPF trades do not intend to fabricate according to the BIM model, there is not much guarantee that a model aids coordination. BIM for load-bearing CFS framing is typically beneficial, but when BIM lacks other trade involvement, it may not be pragmatic, resulting in a wasted effort.
Panelized Exterior or Interior Panels
(For this article, an unfinished panel is defined as a panel that is either just CFS framing members or CFS framing members with sheathing installed.)
Utilizing BIM for prefabricated unfinished CFS panels allows for preliminary coordination of stud layouts, panel joints, panel interactions with other components, and panel connection points to the structure. Minimizing field adjustments during installation is paramount to the success of prefabrication.
A huge benefit of prefabrication is the speed of installation. Utilizing CFS BIM and MEPF coordination before panel fabrication reduces the need for field adjustments and ensures the most efficient installation process for both CFS contractors and MEPF trades.
Note: Coordination with every MEPF item is usually unnecessary for unfinished panels, as some adjustments in the field are acceptable. For example, sheathing can easily be cut in the field, and intermediate framing can be added to support MEPF systems that fit within a stud bay.
Panelized Exterior Finish Panels
(For this article, an exterior finish panel is defined as a panel that includes the air/weather barrier and exterior finish materials in addition to the framing and sheathing.)
BIM for CFS exterior finish panels is usually necessary due to the precision required to properly design and locate all penetrations and attachments of anything that interfaces with the finish panel. As the air/weather barrier is usually covered by finish and no longer accessible, extreme care must be taken to ensure the air/weather barrier remains intact and functional to prevent performance issues. Items such as security cameras, light fixtures, louvers, vents or grilles, keypads, signage, and overflow scuppers are commonly occurring items that require coordination. BIM provides an excellent opportunity to ensure these items are properly located and installed.
Note: Not utilizing BIM for exterior finish panels is risky and can lead to costly field repairs that could compromise the appearance or performance of the exterior envelope.
Panelized Load-Bearing Structures
Load-bearing structural CFS panels can be installed at impressive rates, especially when multiple cranes are used. Some projects have 50% of the CFS panels fabricated before the first panel is set to keep up with this pace. Since issues in the field do not appear until most panels are fabricated, it is critical to utilize BIM to find and resolve conflicts before panel fabrication. Field modification of panelized CFS framing can have a detrimental effect on a project. Even moving one stud has the potential to cascade into a series of misalignments on levels above or below. Thus, CFS BIM for panelized load-bearing structures is almost always a necessity.
Recent Advancements
Cutting-edge integrations have allowed contractors to harness the ability to efficiently build directly from BIM with amazing results and accuracy. Examples of these advancements include CNC roll formers and robotic assembly lines. Information from BIM is exchanged with the equipment via output files. Output files are essentially digital instructions given to equipment so material can be formed, cut to length, and even printed with labels for assembly purposes. Output files can also control fully automated robotic assembly lines that assemble, screw, or weld members into panel assemblies.
Though CNC roll formers and robots are precision pieces of equipment designed to handle immense work volumes, downfalls exist with this technology. The equipment and software can be expensive to own and operate. Robots may struggle to accommodate fit-up tolerances of material causing problems during panel assembly. Machines may also need to run slower to achieve adequate accuracy and may jam while manufacturing complex or short pieces.
Final Considerations
As computers and software continue to improve, so will the benefits of BIM. However, heavy upfront modeling and coordination are required and generate costs that must be addressed by the parties involved. Therefore, it is important to remember the end-use of CFS BIM and how it relates to the CFS framing at the job site. When CFS framing is not panelized, or the CFS contractor does not intend to use BIM during installation, a full-scale CFS BIM effort may be a waste of time and money. However, the benefits can outweigh the costs when CFS BIM is utilized intelligently.■